Loudspeakers are devices to convert electrical energy into acoustical energy. However, much of the electrical power that is applied to the loudspeaker results in heat dissipation, which causes many of the common loudspeaker defects. To prevent thermal damage (permanent or transitory), it is desirable to measure the voice coil temperature of the loudspeaker, and to condition the input in such a way that this temperature does not exceed a certain limit.
One way to estimate the voice coil temperature is to predict its value from the electrical signal that is sent to the loudspeaker using a mathematical model of the loudspeaker using a number of pre-estimated parameters, see e.g., Klippel, W., 2004. “Nonlinear Modeling of the Heat Transfer in Loudspeakers”. J. Audio Eng. Soc. 52, 3-25.
A different approach is to measure the current and voltage in the voice coil directly and estimate its temperature based on those measurements. This approach is taken in Behler, Gottfried; Spätling, U.; Arimont, T., February 1995, “Measuring the Loudspeaker's Impedance During Operation for the Evaluation of the Voice Coil Temperature”, in: Proceedings of the 98th AES Convention, Paris. Paper number 4001. This document will be referred to as Behler 1995.
From the measured voltage and current the DC resistance of the loudspeaker, also referred to as Re, is determined. The DC resistance is estimated as the average of the real part of the impedance for frequencies in the vicinity of the minimum impedance exceeding the resonant frequency of the loudspeaker. Since the DC resistance depends on the temperature of the voice coil, one may determine the temperature from the DC resistance.
Also U.S. Pat. No. 6,940,981, with title “Apparatus and method of limiting power applied to a loudspeaker” takes this approach. The patent discloses a method of limiting the power applied to a loudspeaker. In the method both the voltage and current applied to the loudspeaker are measured and instantaneous power is directly calculated and used to control the level of the input signal that drives the amplifier powering the loudspeaker. When the power applied to the loudspeaker exceeds a prescribed threshold, the input level to the power amplifier is reduced until the measured power falls below the threshold. Also disclosed is a method for indirectly determining the voice coil temperature from the loudspeaker's voltage and current and reducing power to the loudspeaker when the temperature exceeds a prescribed threshold. This document will be referred to as Neunaber, 2005.